How to make toon shader more realistic and clean crooked shadow in models?
One thing that might help is to bump up the subdivision modifier (assuming you have it on), though that might increase render times.

Another thing you might try is custom normals. This way, you can have more control over how the shading looks but it adds several steps to your workflow.

I would also suggest this video. Its long and has nothing to do with blender, but it is very informative.